Coping with NE5532D output DC offset - diyAudio
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Old 17th September 2005, 08:39 PM   #1
VEC7OR is offline VEC7OR  Lithuania
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Default Coping with NE5532D output DC offset

I've designed and assembled a headphone amplifier based on NE5532D (http://images.bite.lt/banga/files/cl...ac13923b17.png schematics, http://images.bite.lt/banga/files/cl...ac189633db.jpg PCB, http://images.bite.lt/banga/files/cl...ac1d14c818.jpg insides, http://images.bite.lt/banga/files/cl...ac1e4eedd1.jpg front)

On each output it has 0.121V DC offset, I dont think it does alot of harm to my 64ohm headphones because power sink due to dc offset is just ~220uW, but it somewhat bugs me, how can I rid of it ?

P.S. As for the amp itself I really like the sound - its dead quiet when there is no signal and transparent when it is .
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Old 17th September 2005, 08:59 PM   #2
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You have made one or two mistakes.

The offset (taken from my head) is 1-4 mV times your gain, 11 in your case (a bit much gain?) this gives you approx. 40-50 mV. The reason why you have so much additional offset is the 100 k at the input. If you want to minimize the offset you should insert a capacitor between 10 k and ground. This will create an offset of 4-10 mV only. The trick is that each input of the opamp should "see" the same DC resistance.
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Old 17th September 2005, 09:12 PM   #3
VEC7OR is offline VEC7OR  Lithuania
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Or I just remove 100k resistor ?
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Old 17th September 2005, 10:00 PM   #4
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Quote:
Originally posted by VEC7OR
Or I just remove 100k resistor ?
Noo!

you have to aim for equal (dc) resistances in both inputs, if you remove that 100k then you end up with far worse situation as then its 10k||100k(~9k) for -input and infinite for + input.
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Old 18th September 2005, 07:18 AM   #5
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Quote:
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Or I just remove 100k resistor ?
No, because every opamp MUST have a DC path somewhere in order to work. The NE5532 has NPN transistors at the input and those transistors must get base current from somewhere.
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Old 21st September 2005, 01:31 PM   #6
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I think I got it now, I should replace 100k resistor in input to 9k resistor, that way both inputs will see the ground through 10k resistance.
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